(1) Technical Field
This invention is concerned with improving the capability and extending the useful features of electromechanical acoustic emission detectors, for detecting acoustic emission signals that are emitted by solid objects with those signals being caused, for example, by material cracking and converting the signals obtained into electrical signals that replicate closely the actual dynamic displacements producing said detection. The transducer described herein responds to a very specific displacement, that is a displacement associated with tangential motion only. The object of this invention is to provide a broadband detector for dynamic tangential surface motion. Another object of this invention is to provide a broadband detector that acts as a point receiver for dynamic tangential surface motion. Its sensitivity has the inverse distance relationship expected of a point receiver. Another object of this invention is to provide a broadband measuring transducer that acts as a point receiver for dynamic tangential surface displacement which can be calibrated to measure absolute tangential surface displacement.
(2) Background Art
Acoustic emission, to which the present invention particularly applies, is concerned with the detection of elastic waves that are emitted from some source located within a solid object and becomes manifest at surfaces remote from the source. Several sources can be emitting at the same time. Often acoustic emission signals occur as a result of crack growth when external stresses are applied to the object that contain the crack. Examples of such stress induced acoustic emission signals can be found in pressure vessels under pressure test or in operation, in welded joints that support some force load while in service, as well as fatigue cracking that may be generated in a structure by a dynamically varying load. Chemical changes or temperature differences associated with this object may also induce acoustic emission.
Traditional acoustic emission transducers in use similar to those described in U.S. Pat. Nos. 3,855,847 and 4,011,472 do not produce voltage outputs that are related to some specified physical quantity, such as dynamic displacement. They are usually resonant in character, having sensitivities that change radically over the nominal frequency range of interest.
Besides questionable frequency response and response to unknown physical quantities, traditional acoustic emission transducers may respond to an unknown sum of different directional components of the surface motion. For instance, these transducers may have sensitivity to both normal and tangential motion, a situation that further confuses the interpretation of the transducer's voltage output. (Normal or tangential motion is motion that is in a direction that is respectively perpendicular to or parallel to the surface at the point of interest.) Another problem that most traditional acoustic emission transducers suffer from is related to the extended contact face. This contact face represents an aperture through which the stress wave energy must pass and as such gives rise to an interference problem for vibrational signals impinging on the front face of the transducer from off-axis positions. This adversely affects both the frequency bandwidth and the spacial sensitivity in a way that is both complicated difficult to assess. Thus traditional acoustic emission transducers are detectors of mechanical motion only and can not be used to obtain quantitative measurement of actual physical displacement or velocity.
Instead of being confined to doing triangulation or inerring the general size of the acoustic emission event from signals that are highly confused by transducer resonances and questionable mode of operation, there is a trend now towards trying to measure the actual valve of a specific physical quantity, for example, displacement or velocity of the surface. A few new transducers have been designed to produce voltage outputs that are faithful reproductions of the normal displacement of the surface over a very wide frequency range. But these transducers measure the normal component of the displacement, while the subject transducer measures the tangential component only.
Because acoustic emission signals come from any position in the soild object of interest, signals arriving at the receiving transducer come by various kinds of wave energy or modes of vibrations. There is a mode that travels along the surface which is known by the names Rayleigh or Lamb. There is a vibrational mode that comes by body waves in which the local displacement is parallel to the path of travel of the wave; this is known as the compressional mode. There is a vibrational mode that comes by body waves in which the local displacements are at right angles to the path of travel of the wave; this is known as the shear mode. The wave energy of each mode travels with a different and unique speed; a distinguishing feature. In the general case, at the location of the receiving transducer, the surface would experience the effects of all the impinging modes of vibration. The complex time-vibrational motion that occurs at one point on the surface contains information about the geometry of the body, about the size and kind of an acoustic emission event, and about the acoustic characteristics of the body media.
If the transducer were a perfec device it would sense the surface motion in its complex form ie. a sum of all the different modes of vibration and from all the different paths leading from the acoustic emission event to the receiving transducer position. This includes single and multi reflection paths as well as combinations of body waves where mode conversion is present. It also includes elastic energy that travels along a path that is contained in the surface. If the performance of the transducer is dominated by the unknown response characteristics alluded to in the previous paragraphs, the output becomes so complicated and clouded with the unknown character of the transducer that any interpertation of direct physical quantities such as surface displacement becomes virtually impossible. The ideal acoustic emission detection needs to faithfully measure one known physical quantity, such as displacement, over a wide frequency range that includes the frequencies of interest and it should be known to be sensitive to motion along one component only of the principal directions of the surface such as normal or tangential directions. The directional and spatial sensitivity should vary in a known and well controlled manner. To date no transducer design exists that can provide a faithful voltage representation of the tangential motion of a point on the surface of a solid. It is to this application that the present invention is directed.